Kinase inhibitors are rapidly becoming one of the most validated and pursued class of drug targets. Janus kinases (JAKs) comprise a family of 4 kinases that play multiple roles downstream of cytokine signalling in both immune and non-immune cells. Autoimmunity is driven by an aberrant adaptive immune response to self-antigens and JAK-STAT signalling is known to play a key role in this process. Thus JAK inhibitors have considerable potential for the development of drugs to treat autoimmunity. JAK3 is an especially attractive target as, unlike other JAKs, its expression is restricted to the immune system. Several pharmaceutical companies have considerable efforts underway to develop JAK3 inhibitors. While the FDA recently approved the JAK3 inhibitor Tofacitnib (XELJANZ®), which is actually a pan-JAK inhibitor, it was rejected by CHMP in Europe due to safety concerns. This highlights the fact that while pan-JAK inhibitors are effective in RA, they carry a range of adverse side effects, many of which may relate to the functions of JAKs outside the adaptive immune system. Therefore, development of selective small molecule inhibitors of JAK3 would potentially represent a promising class of novel therapeutics.